Tunable single-mode laser diode sources have potential uses in many areas including spectroscopy, coherent communication, and nonlinear frequency conversion. Particularly in the area of second harmonic frequency conversion, also known as second harmonic generation or SHG, the narrow band nature of this process dictates the need for a narrow band tunable pump source which can maintain optimum efficiency as a result of drifts in temperature, wavelength, or to compensate for device fabrication errors. If these optical frequency converting devices (laser diode pump source combined with a SHG device) are to be used as a means to convert invisible infrared radiation to visible radiation as would be their use as the source in optical disk recorders, printers, or display devices then light output stability, compactness, and low cost are important additional factors.
Standard commercially available single-mode laser diodes sometimes referred to as Fabry-Perot laser diodes can be tuned by varying their temperature or injection current or a combination of both. However, tuning a laser diode by these methods often leads to undesirable effects. For example, although a large range of tuning may be obtained by varying the temperature of the laser diode the tuning will be interrupted periodically by shifts in the lasing frequency. These frequency shifts or mode hops would be detrimental to the output of a SHG system since the result would be large drops in intensity of the frequency converted radiation. The same detrimental result would occur for injection current tuning due to mode hops as well as the fundamental pump intensity dependence on injection current.
In recent years much research has gone into tunable single-mode laser diodes which are free of these undesirable tuning characteristics. They are generally of two types referred to as Distributed Bragg Reflector (DBR) laser diodes or Distributed Feedback (DFB) laser diodes. Although they differ in structure, in the broadest terms, their spectral characteristics are a result of the incorporation of a grating structure in the cavity. The laser diodes are tuned by an independent injection current which modifies the index of refraction in the grating structure via a change in carrier density as a result of the change in current. The spectral and tuning characteristics of these laser diodes as known in the art are well suited for the applications discussed previously. Unfortunately, they are not as yet commercially available. Although they may be commercially available in the near future their initial cost could prove to be prohibitive.
An alternative to the DBR or DFB laser diodes involves the use of commercially available low cost Fabry-Perot laser diodes. These laser diodes can be made tunable through the utilization of an external optical feedback means. It has been known for some time that optical feedback can be used to control the oscillation frequency of a laser diode. (See R. W. Tkach and A. R. Chraplyvy, J. Lightwave Tech. LT-4, 1655 1986). Devices incorporating this effect referred to as external or extended cavity lasers are well known in the art. They are typically comprised of a Fabry-Perot laser diode with a frequency select portion of the emitted radiation being fed back to the laser diode via a reflective diffraction grating. The diffraction grating is often blazed and used in a Littrow configuration where the first order is reflected back into the laser diode. With enough feedback the laser diode will oscillate or "lock" to the frequency selected by the diffraction grating. By changing the angle of the grating with respect to the incident radiation it is possible to change the frequency selected by the diffraction grating there by tuning the laser diode. In some cases the front facet of the laser diode is partially reflective. In this case the external cavity is a perturbation of the laser diode cavity. In another case an attempt is made to anti-reflection (AR) coat the front facet to eliminate any reflection from this facet. In this case the cavity is incomplete. There is no feedback of radiation from the front facet and the laser diode by itself will not lase. The diffraction grating completes the cavity forming an extended cavity. In practice it is impossible to completely suppress reflections from the front facet resulting in some influence from the internal laser diode cavity. In either case the tuning principle is the same. The individual tuning characteristics differ slightly. For many applications the extended cavity laser scheme is preferred.
The narrow spectral linewidth, low noise, and tunability exhibited by extended cavity lasers make them attractive. Integration of the grating into a waveguide is recognized as a method for possibly reducing the size, weight, mechanical complexity and cost of the source. Such devices employing channel waveguides and etched gratings in glass have been demonstrated. (See D. A. Ackerman, M. I. Dahbura, Y. Shani, C. H. Henry, R. C. Henry, R. C. Kistler, R. F. Kazarinov, and C. Y. Kuo, Appl. Phys. Lett. 58, 449 1991). These devices involved an infrared laser diode source only and were not used for waveguide nonlinear frequency conversion nor did they include a means for frequency tuning.
A popular waveguide nonlinear frequency conversion technique for the SHG of visible light involves the periodic reversing of ferroelectric domains in ferroelectric materials to provide phasematching of the fundamental with the second harmonic. This technique often referred to as quasiphase matching or QPM is well known in the art. A device had been presented which utilized a periodically poled QPM LiNbO.sub.3 waveguide for second-harmonic generation in an attempt to provide automatic QPM by using the periodically poled structure not only as the QPM element but also as a Bragg grating reflector. (K. Shinozaki, T. Fukunaga, K. Watanabe, and T. Kamijoh, J. Appl. Phys. 71, 22 1992). Problems with this technique arise from the fact that the conditions for Bragg reflection and QPM are distinct. As a result, impractical fabrication tolerances are placed on the period of the periodically poled structure. Recently, a device described by U.S. Pat. No. 5,185,752 to Welch and Waarts combines a periodically poled QPM nonlinear waveguide, a Bragg grating structure, and a means for achieving a TM polarization in the QPM waveguide section when coupled to a laser diode. The TM polarization allows use of the highest nonlinear optical coefficient in the most commonly used nonlinear optical materials. The structure described by U.S. Pat. No. 5,185,752 to Welch and Waarts does not include a device for frequency tuning.